It depends how you want your units to react.
Use issue order targetting a point using attack-move to the location of a hero (for instance can be a random one from all the hero or a low hp hero etc) if you want the wave units to attack every unit they meet on the way to their target.
Use issue order targetting a unit using attack, if you want your units to ignore all units except your target.
Hope it helps, ask more specific question if you need more info.